1. Field of the Invention
The present invention relates to a nonwoven reinforcement for a printed wiring base board, a process for producing the same, a printed wiring base board and a printed wiring board.
2. Description of the Related Arts
Glass fiber fabrics have been used over the past years as a nonwoven reinforcement for a printed wiring base board, but said fabrics suffer from the disadvantages of high dielectric constant and high specific gravity. In recent years, investigations have been made on aramid fiber fabrics in the form of liquid crystal, which, however, fail to meet the requirements of a printed wiring base board in electrical insulation properties because of high hygroscopicity.
In such circumstances, there is proposed the use of thermotropic crystalline polyester fiber which is low in dielectric constant, specific gravity and hygroscopicity as a nonwoven reinforcement for a printed wiring base board. For example, there is described a printed wiring base board using woven fabrics comprising thermotropic crystalline polyester fiber as a reinforcement in Japanese Patent Application Laid-Open No. 36892/1987 (Sho-62). However, in the case of producing a thin fabric of thermotropic crystalline polyester fiber, such problems as its low processability, a high manufacturing cost, inferior uniformity of the resultant nonwoven reinforcement and low handling properties such as resin impregnation properties occur.
There is also proposed the use of, as a nonwoven reinforcement, dry system nonwoven fabrics that are obtained by spun lace method (water-jet entangling method). Nevertheless said nonwoven fabrics fail to suffice the requirement of a nonwoven reinforcement for a printed wiring base board by reason of their poor mechanical performance, low uniformity and evils such as unevenness in thickness which is further enlarged as the a fabric is made thin.
In contrast to the foregoing, wet system nonwoven fabrics (fabrics by sheet-making process) have not only excellent mechanical performance but also high uniformity and besides can be made into nonwoven fabrics free from unevenness even for a thin sheet. For instance, there is proposed paper composed of thermotropic crystalline polyester short fiber and thermotropic crystalline polyester pulp in Japanese Patent Application Laid-Open Nos.47818/1995 (Hei-7) and 170295/1996 (Hei-8).
However, with any of the previous conventional methods it has been far from possible to obtain a nonwoven reinforcement for a printed wiring base board which is excellent in various performances such as uniformity resin impregnation properties, mechanical performance and heat resistance.
In general, heat calendering is carried out in order to enhance the mechanical performance and uniformity of nonwoven fabrics. Nevertheless, the practice of the aforesaid method has caused such a problem that the surfaces of the nonwoven fabric are pressed into the form of film, the pores that are capable of being impregnated with a resin are micronized and also the number of said pores are remarkably decreased, thereby making it difficult to impregnate the nonwoven fabric in whole (deterioration of the resin impregnation properties) and consequently forming a portion where the resin is not impregnated into the nonwoven fabric inside. The portion which is non-impregnated with a resin (air), when present to a large extent, brings about at the time of moisture absorption, unstable electrical insulation properties, inferior heat resistance of a solder and an unfavorable nonwoven reinforcement which is required for advanced performances for a printed wiring base board. Notwithstanding the foregoing, unless a heating pressurizing treatment is carried out, it is impossible to maintain the strength which is endurable to the production process such as resin impregnation step and in addition, a problem arises in dimensional stability.
Although an investigation is made on the production of a printed wiring base board with a fiber-reinforced resin by mixing reinforcing fiber in a resin, the reinforcing fiber is difficult to uniformly disperse in a resin. In addition, the reinforcement effect is restricted by the fiber oriented in random directions.